The "rosy" region of chromosome 3 of Drosophila melanogaster controls the biosynthesis of the enzyme xanthine dehydrogenase. This proposal requests support for the continuation of an investigation designed to dissect the component parts of this genetic unit both in terms of structure and function. Experiments, thus far completed, have provided an outline of two components of the system, (a) a coding element responsible for the amino acid sequence of the XDH peptide, and (b) a cis-acting control element contiguous with the coding element. Experiments proposed are: 1. Mutagenesis experiments designed to produce several classes of control variants that map to the rosy locus. 2. Mutagenesis experiments designed to produce several classes of variants that map elsewhere in the genome, and which are alterations in genes involved in transcriptional or pre-translational regulation of XDH. 3. Genetic, biochemical, immunological and developmental studies designed to characterize the variants uncovered in 2 and 3. 4. A study of position effect variegation at the genetic and biochemical levels as background to eventual study at the molecular level. 5. Relate the physical boundaries of the coding element to the genetic boundaries by making use of chemical cleavage experiments and mapping of XDH variants to N and C terminal fragments.